August 25, 2014

Honeybees are more genetically diverse than originally thought, and the species might have originated from Asia and not Africa as previously believed, according to new research published online Sunday in the journal Nature Genetics.

As part of their study, researchers from the Uppsala University Department of Medical Biochemistry and Microbiology and an international team of colleagues present the first global analysis of genome variation in the honeybee (Apis mellifera) – an insect that is partially responsible for pollinating one-third of our fruits, nuts and vegetables.

Despite their importance to global food supplies, however, there is great concern over the extensive loss of honeybee colonies that has taken place in recent years. Honeybees are threatened by disease, climate change and management practices, the authors said, and it is essential to gain a better understanding of their evolutionary history and learn how they managed to adapt to different environments in order to combat these threats.

“We have used state-of-the-art high-throughput genomics to address these questions, and have identified high levels of genetic diversity in honeybees,” researcher Matthew Webster of the university’s Science for Life Laboratory, said in a statement.

“In contrast to other domestic species, management of honeybees seems to have increased levels of genetic variation by mixing bees from different parts of the world,” he added. “The findings may also indicate that high levels of inbreeding are not a major cause of global colony losses.”

In addition, Webster and his co-authors found that honeybees do not appear to be of African origin, as suggested in previous studies. Instead, the new study suggests that they appear to be derived from an ancient lineage of cavity-nesting bees from Asia that began spreading through Europe and Africa approximately 30,000 years ago.

They found evidence suggesting large cyclical fluctuations in population size that mirror historical patterns of glaciation hidden in the patterns of genome variation of the insects – a discovery indicating that honeybee populations had been greatly affected by climate change throughout their history.

“The evolutionary tree we constructed from genome sequences does not support an origin in Africa, this gives us new insight into how honeybees spread and became adapted to habitats across the world,” Webster said. "Populations in Europe appear to have contracted during ice ages whereas African populations have expanded at those times, suggesting that environmental conditions there were more favorable.”

He and his fellow investigators also identified specific genetic mutations important to adaptation to factors such as climate and pathogens, including those involved in morphology, behavior and innate immunity. Webster said that their findings offer new insight into the evolution and genetic adaptation of the bees, while also creating a framework for studying the biological mechanisms behind disease resistance and adaptation to climate.

“We find evidence that population sizes have fluctuated greatly, mirroring historical fluctuations in climate, although contemporary populations have high genetic diversity, indicating the absence of domestication bottlenecks,” the authors wrote. “Levels of genetic variation are strongly shaped by natural selection and are highly correlated with patterns of gene expression and DNA methylation.”

“We identify genomic signatures of local adaptation, which are enriched in genes expressed in workers and in immune system- and sperm motility- related genes that might underlie geographic variation in reproduction, dispersal and disease resistance,” they added. “This study provides a framework for future investigations into responses to pathogens and climate change in honeybees.”